1. Field of the Invention
The present invention relates to a solid-state image pickup device such as a video camera and the like having a photoelectric conversion unit.
2. Description of the Prior Art
In recent years, there have been widely used solid-state image pickup devices as image pickup devices such as video cameras and the like. A solid-state image pickup device has various advantages of being light in weight and compact, having a uniform spatial resolution throughout a whole part of a screen and low in afterimage and so on. However, as there is an increasing need for high image quality through such as EDTVs and HDTVs and removal of false signals represented by moire patterns such as beat noises, enhancement of high resolution is still a problem to be solved.
FIG. 2(a) shows a block diagram of a conventional solid-state image pickup device. FIG. 2(b) shows a color filter array 46 in the conventional solid-state image pickup device, in which the first row is a repetition of magenta (Mg) and green (G), the second row is a repetition of yellow (Ye) and cyan (Cy), the third row is a color filter arrangement in the reverse order to that of the first row, and the fourth row is a color filter arrangement same as that of the second row.
Referring to FIGS. 2(a) and 2(b), incident light passing through the color filter array 46 is subjected to photoelectric conversion in a photoelectric conversion unit 40. The output signal of the photoelectric conversion means 40 is developed with electric charges on the first and second rows added and, likewise, the electric charges on the third and fourth rows added in a first field. In a second field, electric charges on the second and third rows are added and developed. In the photoelectric conversion unit 40, a photodiode mix (PDMIX) is performed by usually adding two vertical pixels to read out electric charges. For example, in FIG. 2(b), an interlace scanning system is adopted in such a manner that the first and second rows as well as the third and fourth rows are added together to be developed in the first field whereas the second and third rows as well as the fourth and fifth rows are added to be developed in the second field.
The output signal of the photoelectric conversion unit 40 is fed to a luminance signal processing unit 41 and a chrominance signal processing unit 42. In the luminance signal processing unit 41, adjacent horizontal signals are added together to thereby produce a luminance signal to be generated at a first output terminal 43. In the chrominance signal processing unit 42, adjacent horizontal signals are subtracted to each other to thereby produce a first color difference signal such as a B-Y signal to be generated at a second output terminal 44 and a second color difference signal such as a R-Y signal to be generated at a third output terminal 45.
Although the conventional solid-state image pickup device has such an advantage as mentioned above that the signal processing is simple, there is a problem that it is insufficient for providing a good resolution with removal of a moire pattern.
FIG. 9(c) shows a two-dimensional frequency characteristic obtained when the conventional solid-state image pickup device is used. In FIG. 9(c), the horizontal axis and vertical axis represent a horizontal frequency and vertical frequency, respectively. As shown in FIG. 9(c), the moire pattern will occur at coordinates of (Nyquist, 0 line), (Nyquist, 250 lines), (Nyquist, 500 lines), (0, 250 lines), and (0, 500 lines). It is to be noted here that Nyquist denotes a frequency (1/2.times.f.sub.s), where f.sub.s represents a sampling frequency.
Moreover, with respect to the horizontal Nyquist frequency, the horizontal luminance signal band is degraded to 0.85 times because it is limited in band width by means of an optical low-pass filter such as a crystal filter and further because adjacent horizontal signals are added to thereby produce a luminance signal in the luminance signal processing procedure. As to the vertical luminance signal, there is also a problem that only a resolution as fine as 350 lines can be obtained after interlace scanning because a vertical resolution of 500 lines is limited in band width by means of an optical low-pass filter and further because adjacent vertical pixels are added in the photoelectric conversion unit 40.